We provide the first direct evidence that highly purified single-walled carbon nanotubes (SWNTs) exhibit strong antimicrobial activity. By using a pristine SWNT with a narrow diameter distribution, we demonstrate that cell membrane damage resulting from direct contact with SWNT aggregates is the likely mechanism leading to bacterial cell death. This finding may be useful in the application of SWNTs as building blocks for antimicrobial materials.Carbon-based nanomaterials, such as fullerene and carbon nanotubes (CNTs), exhibit unique size-and structure-dependent optical, electronic, magnetic, thermal, chemical, and mechanical properties. 1 As a result, it is not surprising that these nanomaterials have been considered for use in numerous applications, including the fabrication of superconductors, optical devices, sensors, energy storage devices, fuel cells, and catalysts. 2 However, such extraordinary physical and chemical properties are accompanied by concerns about possible adverse effects of these materials on biological systems. In particular, applications that use singlewalled carbon nanotubes (SWNTs) for biosensors, 3 drug and vaccine delivery transporters, 4,5 and novel biomaterials 6 increase the potential for encounters between SWNTs and humans and the ecosystem. Future commercial development of nanotechnology may also lead to the discharge of SWNTs into the environment.Information concerning the potential toxicity from exposure to SWNTs and their environmental impact is scarce, often debated, 7 and focused on human cells. [8][9][10][11] The toxicity of SWNTs to human cells has been observed to vary with SWNT functionalization and the concentration of the solubilizing agent (i.e., surfactants), 12,13 as well as with SWNT physicochemical properties such as structure, diameter, cleanliness (e.g., % metal), and defect level. 14 In these studies, however, commercial SWNTs were used, and characterization was not well defined. Furthermore, for these SWNTs, factors such as mean diameter, diameter distribution, metal content, and defect level cannot be independently varied because of limitations of the SWNT synthesis, cleaning, and separation processes. Commercial SWNTs have generally been treated with strong acids and contain on average 4.5-15% metal and other impurities. 15 Only a handful of groups can produce SWNTs with a narrow diameter distribution. 16 The above studies with human cells may suggest that SWNTs can also interact with microbes and exhibit antimicrobial